Phased array antennas are electronically controlled scanning beam antennas including phase shifters or delay lines, usually tunable by electronic or electromechanical means, that provide a differential phase shift or delay on the signals feeding adjacent antenna elements or groups of elements.
DDD technique is a currently used technique for improving performance of wireless communication systems, in particular in downlink direction, by adding a delay diversity to the space and/or polarization diversity provided by transmitting antenna arrays. In other words, different elements in the array transmit differently delayed replicas of the same signal. At a receiver, the differently delayed replicas give rise to alternate constructive and destructive combinations. In the DDD technique, the delays are time-varying and are obtained by tunable delay lines connected in the signal paths towards different antenna elements.
A wireless communication system exploiting the DDD technique is disclosed for instance in WO 2006/037364 A.
Assuming for sake of simplicity that the signals to be delayed can be considered single-frequency signals, so that applying a time delay is equivalent to applying a phase shift, a delay line with length L introduces a phase shift φ=−β·L, or a delay τ=L*dβ/dω, on the signal propagating through it, β being the propagation constant of the line and ω being the signal angular frequency. Thus, in order to vary the phase shift (or the delay), either β or L is to be varied. The most commonly used solutions rely on a variation of β.
Several tunable delay lines based on the variation of β are known in the art and are commercially available. A class of such delay lines rely upon the variation of the position of a dielectric or metal member within the waveguide cavity.
The paper “Dielectric Based Frequency Agile Microwave Devices”, by Y. Poplavko, V. Kazmirenko, Y. Prokopenko, M. Jeong, and S. Baik, presented at the 15th International Conference on Microwaves, Radar and Wireless Communications, 2004, MIKON-2004, pages 828-831, discloses a tunable phase-shifter consisting of a partially dielectrically filled waveguide, where two dielectric plates are placed inside the waveguide and the phase shifting tuning is obtained by changing the width of the air gap between said dielectric plates, thereby controlling the effective dielectric constant ∈eff of the structure. This is obtained by moving at least one of the two dielectric plates up and down by means of a piezoelectric actuator. The waveguide structure has fixed impedance matching sections.
US 2003/0042997 A1 discloses a tunable phase-shifter consisting of a partially dielectric filled waveguide having an air-dielectric sandwich structure comprising either two dielectric members or a dielectric member and a metal plate separated by an air gap. The tuning of the phase shifting is obtained by changing the width of the air gap by moving either at least one or the dielectric members, or at least one out of the dielectric member and the metal plate, by means of a piezoelectric actuator.
The paper “Partially Dielectric-Slab-Filled Waveguide Phase Shifter”, by C. T. M. Chang, IEEE Transactions on Microwave Theory and Techniques, Vol. MTT-22, No. 5, May 1974, pages 481-485, discloses a possible way to optimize matching between a dielectric slab-filled waveguide and an unloaded waveguide. An intermediate block is inserted between the dielectric slab-filled waveguide and the unloaded waveguide, which is a dielectric slab of an opportune width. Experimental results show that VSWR (Voltage Standing Wave Ratio) is kept to less than 1.15 (|S11|>23 dB).
The paper “An Approximate Analysis of Dielectric-Ridge Loaded Waveguide”, R. M. Arnold and F. J. Rosenbaum, IEEE Transactions on Microwave Theory and Techniques, Oct. 1972, pages 699-701, analyzes the behavior of a waveguide partially filled with a dielectric slab. By varying the filling ratio it is possible to control the phase shift of an electromagnetic signal propagating inside the waveguide. The maximum phase shift obtained is about 100° between the case in which the dielectric slab is totally inserted into the waveguide and the case in which the dielectric slab is flush with the waveguide wall.
PCT patent application PCT/EP2006/005202, published as WO 2007137610, discloses a tunable delay line including a ridge waveguide with a dielectric perturbing member separated by a small air gap from a longitudinal end surface of the ridge and movable relative to the ridge for varying the width of the air gap and hence the propagation characteristics of the guide and the delay imparted by the line.